1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrostatic photocopier, a laser printer, or the like in which a toner image is fixed to a fixing material.
2. Description of the Related Art
In a fixing device incorporated in a photocopier using an electrophotographic process, a developer which is toner formed on a fixing material is heated and melted to fix the toner to the fixing material. A method of using radiated heat based on a halogen lamp (a filament lamp) is widely used as a method for heating toner usable for a fixing device.
With respect to the method of using a halogen lamp as a heat source, a structure is widely used, i.e., paired rollers are provided such that a predetermined pressure can be applied to the fixing material and toner, at least one of the paired rollers is used as a hollow column, and a halogen lamp arranged in a column is arranged in the inner hollow space. In this structure, the roller provided with a halogen lamp forms an acting part (nip) at a position where the roller contacts with the other roller, so that pressure and heat are applied to a fixing material and toner guided to the nip. That is, the fixing material, i.e., a paper sheet is passed through a fixing point which is a press contact part between a heat roller provided with a lamp and a press roller which rotates as a slave to the heat roller, and thus, toner on the paper sheet is melted and fixed to the paper sheet.
In the fixing device using a halogen lamp, light and heat from the halogen lamp is radiated in all directions to the entire circumference of the heat roller. In this case, it is known that the thermal conversion efficiency is 60 to 70%, the thermal efficiency is low, the power consumption is large, and the warm-up time is long, in consideration of the loss at the time when light is converted into heat, the efficiency at which air in the roller is warmed to transfer heat to the holler, and the like.
Hence, as a heat source for a heat roller, an induction heating method has been practiced, in which a heat coil is provided inside a heat roller, and a high-frequency current is supplied to the coil, so that heating is carried out by induction heating.
For example, Japanese Patent Application KOKAI Publication No. 59-33476 discloses a technique in which a roller having a thin metal layer on the outer circumference of ceramics cylinder is comprised and an induction current is passed through the thin metal layer of the roller with use of a conductive coil to achieve heating.
Japanese Patent Application KOKAI Publication No. 258586 discloses a method which uses a heat generation member in which a coil is wound around a core provided along the rotation axis of a fixing roller, and which achieves heating by flowing eddy current through the fixing roller.
Since the induction heating heats a roller by eddy current obtained as a result of flowing current through a coil, a large electric power is required to heat a heat roller to a predetermined temperature in a short time period.
However, a fixing device used in a photocopier has an upper limit to the power which can be consumed singly by only the fixing device, and electric power is also consumed by a large number of components constructing the photocopier. It is therefore known that a large electric power cannot be continuously supplied only to the fixing device.
Therefore, if a large electric power cannot be distributed to heating of the heat roller of the fixing device, the warm-up time of the photocopier is elongated, so that the time required for obtaining a copy is also elongated. If priority is given to warm-up of the photocopier, the fixing rate is insufficient in some cases. Meanwhile, in a heat roller having a structure in which the heat roller is formed into a thin cylinder made of metal and a coil is provided inside the cylinder along the axial direction of the cylinder, an irregular temperature distribution is caused on the outer circumferential surface of the roller. Therefore, the heat roller must be rotated in contact with the press roller when the temperature of the heat roller increases, to make uniform the temperature of the outer circumferential surface of the heat roller. This lengthens not only the warm-up time but also the electric power required for heating.